A membrane touch switch is a momentary contact, low voltage switch which includes a first and a second switching element or membrane, disposed on opposite sides of an insulating membrane. The insulative membrane has appropriate apertures through which electrical contact between the switching elements may be made. Each switching element has a layer of conducting material formed in a predetermined circuit pattern on one surface thereof. One of the switching elements is connected to a source of electrical potential and the other to ground potential. An electrical circuit is completed when one of the switching elements is depressed and thus extended through the aperture in the insulating membrane to bring the conducting material disposed on the surface of that switching element into contact with the conducting material on the other switching element.
Typically, each of the switching elements includes a substrate of a flexible material, for example, a polyester film such as that sold by E. I. du Pont de Nemours and Company, Inc. under the trademark Mylar.RTM.. Of course, other flexible materials, such as polycarbonate film or polyvinyl fluoride (PVF) film may be used as the flexible substrate. In some instances, the substrate of the upper switching elements is flexible, with the substrate of the base switching element being rigid.
The circuit pattern is usually formed on the switching element by applying onto the substrate a thick film paste composition containing particles of a conductive material dispersed in a volatile organic solvent-resin medium. After screen printing the composition is dried, ordinarily by heating, and the volatile organic material in the solvent-resin medium is driven off. The resin is left to bind together the conductive particles, thus forming the electrically conducting circuit pattern on the switching element.
Membrane touch switches are fabricated using either a semiautomatic or an automatic high speed (e.g., reel-to-reel) technique. In the semiautomatic processing technique, the substrate is manually fed to a printing apparatus and the composition is screen printed onto the surface of the substrate. In the automatic high speed reel-to-reel technique, a roll of substrate material is passed through an automatic printing station and through a drying station before being taken up on suitable take-up reels. The composition is automatically screen printed and dried.
The technology surrounding membrane touch switches in general, and the compositions therefor in particular, are continuously changing to satisfy more stringent environmental requirements, to provide lower operating costs, and to meet changing design criteria and increasing functional requirements imposed on them by the marketplace. Increased demands on paste composition functionality include more durability to harsh environmental changes, increased ability to use the switch at higher temperatures and increased abrasion resistance and increased resistance to high pressure electrical connectors. The most immediate market needs are perceived at this time to be lower processing costs and increased product capabilities. Processing costs can be reduced by providing paste compositions with better conductivity efficiency (i.e., greater conductivity per unit weight of conductive material), less and/or lower cost conductive material and higher throughput capability including longer screen life.
Heretofore, the conductive materials for membrane touch switches have been noble metals, especially silver. However, upward fluctuations in the price of silver metal have made it extremely important to find ways to substitute less expensive conductive materials, such as base metals, without adversely affecting either the primary electrical properties or the secondary functional properties mentioned above.